Products of the ras gene family, termed p21ras, are GTP-binding proteins that have been implicated in signal transduction via receptors encoding tyrosine kinase domains. Recent findings have defined a superfamily of hemopoietin receptors that includes receptors for a number of interleukins and colony-stimulating factors. The intracellular portions of these receptors show only restricted homologies, have no tyrosine kinase domain, and provide no clues to the mode of signal transduction. However, in most cases the factors stimulate tyrosine phosphorylation. We demonstrate here that ligand-induced activation of the interleukin (IL)-2, IL-3, IL-5, and granulocyte-macrophage colony-stimulating factor receptors resulted in activation of p21ras in various hemopoietic cell lines. The only cytokine tested that binds to a hemopoietin receptor and that did not activate p21ras was IL-4. Activation of p21ras was also observed in response to Steel factor, which stimulates the endogenous tyrosine kinase activity of the c-kit receptor, as well as with phorbol esters, which activate protein kinase C. Experiments with protein kinase inhibitors implicated tyrosine kinase activity, but not protein kinase C activity, as the upstream signal in p21ras activation via these growth factor receptors. Attempts to demonstrate tyrosine phosphorylation of the p21ras GTPase-activating protein (GAP) were negative, suggesting that phosphorylation of GAP may not be the major mechanism for regulation of p21ras activity by tyrosine kinases.